Cyclin-dependent kinases and related serine/threonine protein kinases are important cellular enzymes that perform essential functions in regulating cell division and proliferation. The cyclin-dependent kinase catalytic units are activated by regulatory subunits known as cyclins. At least 16 mammalian cyclins have been identified (Johnson D. G. and Walker C. L., Annu. Rev. Pharmacol. Toxicol. 1999;39:295–312). Cyclin B/cdk1, Cyclin A/cdk2, Cyclin E/cdk2, Cyclin D/cdk4, Cyclin D/Cdk6, and probably other heterodimers including Cdk3 and Cdk7 are important regulators of cell cycle progression. Additional functions of Cyclin/Cdk heterodimers include regulation of transcription, DNA repair, differentiation and apoptosis (Morgan D. O., Annu. Rev. Cell. Dev. Biol. 1997; 13261–13291).
Increased activity or temporally abnormal activation of cyclin-dependent kinases has been shown to result in the development of human tumors (Sherr C. J., Science 1996;274:1672–1677). Indeed, human tumor development is commonly associated with alterations in either the Cdk proteins themselves or their regulators (Cordon-Cardo C., Am. J. Pathol. 1995;147:545–560; Karp J. E. and Broder S., Nat. Med. 1995;1:309–320; Hall M. et al., Adv. Cancer Res. 1996;68:67–108). Naturally occurring protein inhibitors of Cdks such as p16 and p27 cause growth inhibition in vitro in lung cancer cell lines (Kamb A., Curr. Top. Microbiol. Immunol. 1998;227:139–148).
Small molecule Cdk inhibitors may also be used in the treatment of cardiovascular disorders such as restenosis and atherosclerosis and other vascular disorders that are due to aberrant cell proliferation. Vascular smooth muscle proliferation and intimal hyperplasia following balloon angioplasty are inhibited by over-expression of the cyclin-dependent kinase inhibitor protein p21 (Chang M. W. et al., J. Clin. Invest., 1995;96:2260; Yang Z-Y. et al., Proc. Natl. Acad. Sci. (USA) 1996;93:9905. Moreover, the purine cdk2 inhibitor CVT-313 (Ki=95 nM) resulted in greater than 80% inhibition of neointima formation in rats (Brooks E. E. et al., J. Biol. Chem. 1997:29207–29211).
Cdk inhibitors can be used to treat diseases caused by a variety of infectious agents, including fungi, protozoan parasites such as Plasmodium falciparum, and DNA and RNA viruses. For example, cyclin-dependent kinases are required for viral replication following infection by herpes simplex virus (HSV) (Schang L. M. et al., J. Virol. 1998;72:5626) and Cdk homologs are known to play essential roles in yeast.
Selective Cdk inhibitors can be used to ameliorate the effects of various autoimmune disorders. Chronic inflammatory disease rheumatoid arthritis is characterized by synovial tissue hyperplasia; inhibition of synovial tissue proliferation should minimize inflammation and prevent joint destruction. Expression of the Cdk inhibitor protein p16 in synovial fibroblasts led to growth inhibition (Taniguchi K. et al., Nat. Med. 1999;5:760–767). Similarly, in a rat model of arthritis, joint swelling was substantially inhibited by treatment with a p16 expressing adenovirus. Cdk inhibitors may be effective against other disorders of cell proliferation including psoriasis (characterized by keratinocyte hyperproliferation), glomerulonephritis, and lupus.
Certain Cdk inhibitors may be useful as chemoprotective agents through their ability to inhibit cell cycle progression of normal untransformed cells (Chen et al. J. Natl. Cancer Institute, 2000;92:1999–2008). Pre-treatment of a cancer patient with a Cdk inhibitor prior to the use of cytotoxic agents can reduce the side effects commonly associated with chemotherapy. Normal proliferating tissues are protected from the cytotoxic effects by the action of the selective Cdk inhibitor.
Review articles on small molecule inhibitors of cyclin dependent kinases have noted the difficulty of identifying compounds that inhibit specific Cdk proteins without inhibiting other enzymes. Thus, despite their potential to treat a variety of diseases, no Cdk inhibitors are currently approved for commercial use (Fischer, P. M., Curr. Opin. Drug Discovery 2001, 4, 623–634; Fry, D. W. & Garrett, M. D. Curr. Opin. Oncologic, Endocrine & Metabolic Invest 2000, 2, 40–59; Webster, K. R. & Kimball, D. Emerging Drugs 2000, 5, 45–59; Sielecki, T. M. et al. J. Med. Chem. 2000, 43, 1–18.).